1. Field of the Invention
This invention relates to a combustor for a gas turbine engine, and more particularly to air scoops for such a combustor having a symmetrical cover to eliminate variations in air flow therethrough caused by the random motion of the air on the outside of the combustor shell.
2. Description of the Prior Art
In gas turbine engines, the combustion chamber (combustor) is generally enclosed in a casing into which compressed air is delivered. The compressed air enters the combustor through openings in the sidewall thereof for primary combustion air, film cooling, dilution and temperature profiling of the exiting combustion gases. However, as the air in the casing is continuously moving in a random manner, its entry into the combustor through any of the various openings is not uniform, but continuously varies for any one such opening and also varies as between similarly sized and positioned openings.
Thus, for the most part, openings into the combustor, at least in cylindrical combustors of the type used in gas turbines of the assignee of the present invention, are placed in annular arrays at certain axial locations thereon to maintain, as closely as possible, symmetry of the combustion process, temperature profiling, wall cooling, etc. within the combustor. However, because of the above random direction and velocity of the air movement around the exterior of the combustor, such symmetry is difficult to obtain.
In the combustion process, it is desirable to have predictability of the air entry in that a certain amount of penetration of the air into the axially entering atomized fuel is necessary for complete combustion of the fuel. Thus, if at times the air flow exteriorly of the combustor has primarily an axial or circumferential flow while at other times or at other combustor air inlets at the same time the flow is primarily radially into the combustor, it is obvious that air penetration is effected, which in turn effects the combustion process, i.e., producing unburned fuel resulting in smoke and high emissions, and producing areas of elevated combustion temperatures adjacent certain areas of the combustor walls and thereby producing thermal stresses in the combustor. Other deleterious effects such as unpredictable startup, nonuniform temperature distribution, etc. also result from such random entry of the air into the combustion zone.
U.S. Pat. Nos. 3,581,492 and 2,916,878 are examples of structure directed to obtain some uniformity to the combustion air entering a combustor. However, it is noted that these structures are not symmetrical and are, for the most part, turning vanes or scoops facing upstream on the assumption the air on the outside of the combustor is flowing axially downstream into the structure. Since the exterior air actually has a swirling random motion, such structure may reduce the nonuniformity problem somewhat, but would not eliminate it.